Re: [PATCH v4 6/9] f2fs: don't allow DIO reads but not DIO writes

[Andreas Dilger]

On Aug 19, 2022, at 5:09 PM, Eric Biggers <ebiggers@xxxxxxxxxx> wrote:
> 
> On Tue, Aug 16, 2022 at 10:42:29AM -0600, Andreas Dilger wrote:
>> 
>> IMHO, this whole discussion is putting the cart before the horse.
>> Changing existing (and useful) IO behavior to accommodate an API that
>> nobody has ever used, and is unlikely to even be widely used, doesn't
>> make sense to me.  Most applications won't check or care about the new
>> DIO size fields, since they've lived this long without statx() returning
>> this info, and will just pick a "large enough" size (4KB, 1MB, whatever)
>> that gives them the performance they need.  They *WILL* care if the app
>> is suddenly unable to read data from a file in ways that have worked for
>> a long time.
>> 
>> Even if apps are modified to check these new DIO size fields, and then
>> try to DIO write to a file in f2fs that doesn't allow it, then f2fs will
>> return an error, which is what it would have done without the statx()
>> changes, so no harm done AFAICS.
>> 
>> Even with a more-complex DIO status return that handles a "direction"
>> field (which IMHO is needlessly complex), there is always the potential
>> for a TOCTOU race where a file changes between checking and access, so
>> the userspace code would need to handle this.
> 
> I'm having trouble making sense of your argument here; you seem to be saying
> that STATX_DIOALIGN isn't useful, so it doesn't matter if we design it
> correctly?  That line of reasoning is concerning, as it's certainly intended
> to be useful, and if it's not useful there's no point in adding it.
> 
> Are there any specific concerns that you have, besides TOCTOU races and the
> lack of support for read-only DIO?

My main concern is disabling useful functionality that exists today to appease
the new DIO size API.  Whether STATX_DIOALIGN will become widely used by
applications or not is hard to say at this point.

If there were separate STATX_DIOREAD and STATX_DIOWRITE flags in the returned
data, and the alignment is provided as it is today, that would be enough IMHO
to address the original use case without significant complexity.

> I don't think that TOCTOU races are a real concern here.  Generally DIO
> constraints would only change if the application doing DIO intentionally does
> something to the file, or if there are changes that involve the filesystem
> being taken offline, e.g. the filesystem being mounted with significantly
> different options or being moved to a different block device.  And, well,
> everything else in stat()/statx() is subject to TOCTOU as well, but is still
> used...

I was thinking of background filesystem operations like compression, LVM
migration to new storage with a different sector size, etc. that may change
the DIO characteristics of the file even while it is open.  Not that I think
this will happen frequently, but it is possible, and applications shouldn't
explode if the DIO parameters change and they get an error.

Cheers, Andreas

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